Is it all text? Is there a file attachment? Embedded images? Most, if not all, email services limit the size of outgoing messages and attachments. You should be able to find what those limits are by visiting the mail service's support web page.
One of the easiest ways to exceed limits is to attach many files to the same message. You may have to send several messages to the same person(s), each with fewer (or one) attachment(s).